If you want a pressure treated deck to last, you should treat it immediately after it's constructed. Now, you will often hear the conventional "wisdom" of needing to let it dry, season, age, or whatever, from many. Fact is, treated lumber is typically soaked with moisture and just waiting to split, curl, twist and shrink as it's allowed to dry in an uncontrolled manner. For many years, I would tell new home customers that they should put some kind of finish on their deck, ASAP, to keep it looking nice. Generally, I was ignored, and by the next year, there were twisted pickets and crooked railings, and a deck that looked a lot older than it was. Eventually, I gave up, and paid the painting contractor to pressure wash and then spray a coat of Thompson's Water Seal on, as the house wash being finished. I actually failed to follow my own advice a few years back, and ignored a new deck on the west side of a new home I built for the wife and I. Two years after I built it, I had to replace all the 2x2 pickets, and much of the handrail, since it was a twisted up mess.
You can either pressure wash newer treated, or use a prep product like Behr All in One Wood Cleaner, from Home Depot, to get the wood clean and get rid of the mill glaze on new wood. Let it dry for a few days, then apply a clear sealer. Thompson's is looked down upon by some, who can't see how something that cost 1/3 of what a top notch stain does, and looks like can of water, could be any good. Well, first, it's a water repellent finish that seals the wood well enough that it won't warp, and twist as much, as it cures in the sun. Second it doesn't build up like a stain or paint, so it's not constantly wearing through in traffic areas. It needs to be reapplied, at best every two years, but it's cheap, takes no prep other than cleaning the dirt off, and it does the job well. Many years ago I covered a very large deck of ours with a very expensive, imported, Semi-solid deck stain. It turned out to be a huge mistake, as it was always wearing thin in foot traffic areas, and needed to be re-coated every 2-3 years anyway. ( When it comes to durability claims, there are more lies on the side of a can of deck stain, than at a campaign rally, "lasts five to seven years on decks, ten on fences" bullshit! ) The biggest issue was that it was a permanent choice. It was essentially a paint, and the deck was going to get coated with some version of the product, at $50/gallon, pretty much forever. The other great part about a clear finish, or a lightly tinted sealer, is that you can change your mind later. If you really want something different, wait two years, pressure wash and do whatever you want next time.